This invention relates to dynamoelectric machines and, more particularly, to determining commutation information for a motor using the DC bus current profile. This profile includes both the current's amplitude and waveshape. While the invention is primarily for use with brushless permanent magnet motors and switched reluctance motors, those skilled in the art will appreciate the wider applicability of the invention for use with other types of motors.
Brushless permanent magnet DC motors, also known as BPM or BLDC motors, require that currentflow through their stator windings be commutated so a desired phase relationship is maintained with respect to the rotor position at any given instant. This commutation is accomplished using an inverter. For most efficient motor operation, it is desirable to commutate the motor at relatively precise moments. This requires the inverter be operated so its switching "on" and "off" of voltage or current to the windings is a function of the position of the rotor. The rotor position with respect to a phase winding, when the winding is energized, is known in the art as the commutation angle. For commutation control purposes, sensors such as Hall effect sensors, or magnetic or optical encoders have been used with motors to sense the rotor's instantaneous position. However, these sensors add to the cost of the motor not only because of their unit cost, but also because of the additional wiring required between the motor and inverter. Sometimes space limitations or environmental factors make it impractical to use these sensors. In such instances, indirect or "sensorless" techniques are employed. Examples of these techniques are disclosed in U.S. Pat. Nos. 4,928,043 to Plunkett, 4,912,378 to Vukosavic, 4,459,519 to Erdman, 4,491,772 to Bitting, 4,743,815 to Gee et al., 4,169,990 also to Erdman (misspelled Lerdman on the face of the patent), and 4,162,435 to Wright. In addition to these patents, we are also aware of the paper by Colby and Novotny on optimizing the efficiency of brushless permanent magnet drives using an open loop system, entitled An Efficiency Optimizing Permanent Magnet Synchronous Motor Drive, Roy S. Colby and Donald W. Novotny, Department of Electrical and Computer Engineering, University of Wisconsin-Madison, 1987; and the paper by Nakamura et al., High-Efficiency Drive Due To Power Factor Control Of A Permanent Magnet Synchronous Motor, Y. Nakamura, T. Kudou, F. Ishibashi, and S. Hibino, IEEE Transactions, 1992.
In Plunkett, a brushless DC permanent magnet motor has an associated feedback loop between its stator windings and an inverter. A timer is used to supply current to the stator windings in a controlled sequence. For commutation purposes, the back EMF (BEMF) of an unenergized winding is sensed and compared with a predetermined value (null point). To maximize motor torque, the switching time of the inverter is controlled by the output of a voltage controlled oscillator (VCO). The VCO is responsive to the difference between amplitude of the BEMF and an optimum amplitude to adjust its output to the inverter. In addition to the added circuitry required by this system, it does not have the flexibility of, for example, a microprocessor based system.
The Vukosavic patent, which is assigned to the same assignee as the present application, discloses a system for determining rotor position also using BEMF. The system uses the third harmonic of BEMF, and obtains this by summing the terminal voltages of the motor. The rotor position is a function of the phase angle of the third harmonic, and commutation is accomplished by switching on current or voltage to a non-energized winding in response to the phase angle reaching a predetermined angle. A microcontroller is usable for this purpose; or, a phase locked loop can also be used. An advantage of this approach is that the third harmonic signal is essentially free of distortions caused by inverter switchings. However, the system requires access to the motor's neutral connection and its operation is an open loop operation during motor starting.
Gee at al., which is also assigned to the same assignee as the present application, describes a control system for a brushless permanent magnet motor. A microprocessor is responsive to zero crossings of motor BEMF to control commutation of the multi-phase motor. Signals are periodically generated indicating the relative position of the rotor relative to the stator. Whenever the rotor position is determined to be at one of a plurality of positions relative to the stator (zero crossing points), an interrupt signal is generated. The microprocessor is responsive to these interrupts to activate switches, thereby permitting current to flow through the respective phase windings. Although the control system is a closed loop system, motor operation must initially be open loop until the motor reaches a predetermined speed.
The Erdman '990 patent teaches a brushless DC motor in which a detecting circuit is used to sense motor BEMF. The detecting circuit integrates the BEMF of the unenergized winding to produce a commutation signal. Commutation signals are produced whenever the integrated signal exceeds a predetermined reference signal, representing rotor position, with which it is compared. The detecting circuit must reset after every comparison. The simulated rotor position signal is referenced to a stationary armature and a predetermined angle of advancement is maintained. A power circuit responds to the derived signal to control application of current to the motor's windings. One problem with this approach is the amount of circuitry required for BEMF detection, integration, current application control, and reset. Another is the inability of this circuitry to control motor operation until it is operating at some minimum speed where the measured BEMF is sufficient to permit the circuitry to function effectively.
In Wright, one winding of the motor is energized. The voltage across a second winding is then sampled and integrated in a manner similar to that in the Erdman '990 patent discussed above. This provides a flux indication which is then compared to a reference value. When the integrated value exceeds the reference value, the next motor winding is energized. The integrated value is simultaneously reset to zero and another cycle of sampling commences. Although measuring the BEMF across the unenergized winding does yield an indication of rotor position, this approach has certain drawbacks. For example, extensive hardware is needed to implement the sampling and integration scheme. Also, there must be an open loop start-up and ramp-up to a minimum operating speed before there is sufficient BEMF signal for the scheme to work. The Erdman U.S. Pat. No. 4,459,519 and Bitting U.S. Pat. No. 4,491,772 patents also disclose integration techniques.
In their paper, Colby and Novotny describe improvements in open loop operations of a brushless DC motor. Besides being open loop, their improvements do not include use of DC current information for control purposes as described hereinafter.
The commutation approach described by Nakamura et al. in their paper referred to above attempts to control the power factor of a motor rather than commutation angle. The circuitry described in this paper includes a sample and hold circuit and measures the difference in amplitude of the DC bus current immediately before, and immediately after, the commutations. To achieve maximum power factor, the circuit attempts to minimize the measured current difference .DELTA.I.sub.DC. It does this by changing the pulse width modulation (PWM) voltage applied to the motor.
Other approaches in sensorless operations include diode conduction and winding inductance. Diode conduction involves detecting current flow during an open phase interval (approximately 60.degree.) of the motor line current waveform. The current flow is caused by the BEMF in the open phase and, if the rotor is aligned properly, starts at the midpoint of the phase (approximately 30.degree.). Motor line current sensors can directly sense this conduction; or, the conduction can be indirectly sensed using free-wheeling diodes. This approach is described in An Approach to Position Sensorless Drive for Brushless dc Motors, by Satoshi Ogasawara and Hirofumi Akagi, IEEE Transactions on Industry Applications, Vol. 27, No. 5, September/October 1991.
The winding inductance approach is premised on the rotor's position being inferred by determining which of a number of windings has the lowest inductance at a given time. High frequency signals are injected into an unenergized winding and resulting peaks are then measured. This approach is shown, for example, in U.S. Pat. Nos. 5,028,852 to Dunfield, and 4,876,491 to Squires et al.